Long lived switch means for inductive dc circuits

ABSTRACT

A switch having unlike metal contacts in a deep vacuum environment has its higher melting point contact connected to be negative at switch opening. In shunt across the switch contacts are a resistor and a capacitor, in series; or a resistor alone can be shunted across the load. In either case the resistor is of about 500 to 5,000 ohms. The capacitor is of a value to afford to the shunt circuit a time constant of not less than about one microsecond.

United States Patent Inventor Appl. No.

Filed Patented Assignee John D. Snnti West Allis, Wis.

Dec. 3, 1970 Nov. 16, 1971 Briggs & Stratton Corporation Wauwnton, Wk.

Continuation-import of application Ser. No. 795,883, Feb. 3, 1969, now abandoned.

LONG LIVED SWITCH MEANS FOR INDUCTIVE DC CIRCUITS 5 Claims, 2 Drawing Figs.

US. (I 307/137, 200/166 C, 3 l 7/DlG. 6

Int. Cl H0lll 9/00 FieldofSearch 3l7/ll.4;

307/93, 136, 137; ZOO/166C [56] References Cited UNITED STATES PATENTS 2,470,825 5/1949 Mathes 3 17/ l 1.4 OTHER REFERENCES Dallas et al. Sensitive Relay Contact Protection Systems" AlEE Technical Paper 48 125, Aug. 1948 Copy in 317/1 1.4.

Primary Examiner-L. T. l-lix Attorney-Ira Milton Jones than about one microsecond.

7 ENVELOPE EVACUATED TO DEEP VACUUM h LOAD TUNGSTEN l2 M PATENTEDuuv 15 I9?! ENVELOPE EVACUATED TO DEEP 9. l'l.

LOAD CIRCUTT MOLYBDENUM CONTACT TUNGSTEN CONTACT FIG.2.

m T Y W R E 0 VH4 T -h T I'D A J LONG LIVED SWITCH MEANS FOR INDUCTIVE DC CIRCUITS This application is a continuation-in-part of my application, Ser. No. 795,883, filed Feb. 3, 1969, now abandoned.

This invention relates to electrical switches having contacts of unlike metals operating in a deep vacuum and intended for the control of inductive and other circuits in which there is an essentially instantaneous rise in voltage across the switch contacts at switch opening and in which cunent always flows in the same direction at the instant of switch opening. A switch of the general type to which the invention relates is disclosed in my copending application, Ser. No. 818,913, filed Apr. 24, I969, for Reed Switch for Rapid Cycle, High Power Applications.

As explained in said copending application, the greater portion, by far, of the wear sustained by the contacts of a switch occurs during an extremely short interval in each opening of the switch when the contacts are just beginning to separate. As contact pressure is relieved, the area of engagement of the contacts rapidly diminishes, until finally they are touching one another only at an infinitesimal point through which all of the current in the circuit must flow. Even at low voltages, the current flowing through this very localized area charges enough energy into the contact metal at that spot to heat it above its melting point, however high this may be. As the contact membersmove apart, the molten particle of metal thus formed is for a brief instant held in place by surface tension, bridging the separating contacts. As separation of the contacts continues, the molten metal particle collapses and settles back onto the contacts. However, the greater portion of the molten metal settles onto the negative contact because the positive contact becomes hotter than the negative one. At each switch opening, therefore, a small amount of metal is thus transferred from the positive contact to the negative contact.

Such transfer of material is known as low-voltage phenomenon. It occurs to a greater or lesser extent in all switches, regardless of the metals of which their contacts are made or the environment in which the contacts operate.

While the amount of metal thus transferred at each switch opening is small, it varies more or less in proportion to the amount of current carried by the switch, and of course it also depends upon the melting point of the metals comprising the contacts. In a switch that is intended to control a circuit in which current always flows in the same direction at the time of switch opening, the low-voltage transfer takes place in the same direction at every opening of the switch, and the material transfer that occurs with successive openings is cumulative.

If the current density through the switch at opening has a high value relative to the vaporizing temperature of the metal comprising the contacts, a further and more active retransfer of material can occur immediately upon collapse of the bridge of molten metal. Under such conditions, a voltage difference appears between the contacts as the bridge collapses, and under its influence an abundance of electrons is emitted by the molten metal on the negative contact. Depending upon circumstances, such electron emission may not develop beyond an incipient arc or cathode spot, or it may develop into a fullfledged plasma are. In either case, the electron emission, due to the potential difference between the contacts, results in ionization of metal on the negative contact, producing a more intensified heating there which causes ionized metal from the negative contact to be emitted into the space between the contacts.

In the switch of the aforesaid application, the contact which is intended to be negative at switch opening is made of a very high melting temperature metal such as tungsten, and the other contact is made of metal having a lower melting temperature such as impure tungsten or (preferably) molybdenum. These con acts are hermetically sealed in an envelope which maintains a deep vacuum environment for them.

With that arrangement. the lower melting metal from the positive contact is transferred to the negative contact by lowvoltage phenomenon during the initial stage of contact separation, and is retransferred back to the positive contact during the immediately subsequent stage of switch opening.

Assuming that the contact which is negative at switch opening is of tungsten and that the positive contact is of molybdenum, only the molybdenum takes part in the initial low-voltage molten metal transfer because the melting temperature of the tungsten is not attained at that time, and moreover, because such transfer takes place in the direction to deposit molybdenum onto the tungsten.

Assuming that conditions are favorable, the tungsten likewise takes no part in the subsequent retransfer. if the power level through the switch at the instant the cathode spot forms is below a certain level, the temperature of that cathode spot will be determined by the characteristics of molybdenum, and once the molybdenum is vaporized off of the tungsten, insufficient energy will be available to effect vaporization of the tungsten, and therefore ionization will cease. Under these conditions, the molybdenum that is deposited on the tungsten negative contact during the initial molten metal transfer will be substantially entirely redeposited on the molybdenum positive contact during the immediately subsequent stage of contact separation, so that neither contact will sustain any substantial loss of metal, even through many successive operations of the switch.

Of course some breakdown of the contacts will eventually occur, because not all of the ionized molybdenum is redeposited on the positive contact (some of it is lost to the walls of the envelope) and because not all of the molybdenum that is redeposited on the positive contact is brought to the exact area thereof from which it was initially transferred in the molten stage.

As indicated above, the transfer and retransfer of the molybdenum without involvement of the tungsten is dependent upon the existence of favorable conditions. However, if the voltage across the contacts can rise to a high enough value so that ionization of the tungsten can take place, then a certain amount of tungsten will be deposited on the molybdenum contact at each switch opening, and after a certain number of switch operations the contacts will come to behave as if they were both of tungsten, with the result that the switch will fail after substantially fewer operating cycles than under favorable conditions.

The voltage that can develop acros the switch contacts during the brief but critical interval of ionization depends upon the characteristics of the electrical circuit in which the switch is connected and also depends upon the conductivity of the contact environment. If the contacts are in a substantially perfect vacuum, the resistance between them as they initially separate can rise to a high value, and if the current through the switch is high enough, and the switch is controlling a circuit in which there is an essentially instantaneous change of voltage across the switch contacts at the instant of switch opening (e.g., an inductive circuit), the voltage across the contacts can rise to the point at which tungsten is vaporized. If, however, the switch envelope is slightly gassy," the gas present around the contacts reduces the resistance between them, and a voltage level sufficient for tungsten ionization may not be attained even though the current through the switch is relatively high.

Thus a certain level of gassiness in the envelope may be desirable insofar as it serves to limit the maximum voltage across the contacts during switch opening to a value high enough for molybdenum vaporization but too low for vaporization of tungsten. However, it is difiicult to achieve precisely a desired level of gassiness in a switch, especially since that level must be a very low one; and even if this were not a problem, a switch having any particular level ofgassiness is suitable only for controlling power of a limited range of values. Moreover, the ionization process encourages chemical combination of the gas with the envelope and the contact metals and absorption of the gas by, the materials of those parts. so that the level of gas in the envelope may change with successive operations of the switch. and the useful life of the switch may be affected accordingly. From a practical standpoint, therefore, gassiness is an undesirable expedient for attaining long useful life in a switch of the character here under consideration.

With the foregoing observations in mind, it is the general object of this invention to provide very simple and inexpensive external circuit means for controlling the maximum voltage that can be developed across the contacts of a switch of the character described, whereby vaporization of the metal of the negative switch contact is prevented, to thus bring about a substantial improvement in the current handling capabilities and useful life of such a switch.

More specifically, it is an object of this invention to provide means cooperable with a switch of the character described to control the maximum voltage across its contacts dun'ng switch opening, and which means is effective only through the very brief critical interval during switch opening in which a cathode spot persists, but otherwise has no appreciable effect upon the characteristics of the circuit in which the switch is incorporated.

With these observations and objects in mind, the manner in which the invention achieves its purpose will be appreciated from the, following description and the accompanying drawing. This disclosure is intended merely to exemplify the invention. The invention is not limited to the particular structure or method disclosed, and changes can be made therein which lie within the scope of the appended claims without departing from the invention.

The drawing illustrates two complete examples of physical embodiments of the invention constructed according to the best modes so far devised for the practical application of the principles thereof, and in which:

FIG. 1 is a more or less diagrammatic representation of a circuit which embodies the invention and which incorporates a switch that is shown mainly in elevation but with portions of its envelope broken away; and

FIG. 2 is a circuit diagram of a modified embodiment of the invention.

Referring now more particularly to the accompanying drawing, the numeral 5 designates generally a switch having an evacuated envelope 6 in which there are a pair of contacts 7 and 8 of unlike metals. As illustrated, the switch 5 is a magnetically actuated switch embodying the principles disclosed in the aforesaid copending application, and it is adapted to be actuated to its closed condition by a magnetic field and to open when the field is removed.

Specifically, the switch 5 has a pair of reed elements 9 and 10, each comprising a rodlike terminal member 11 which extends through a sealed end of the envelope 5, a relatively stiff rebound post 12 which projects inwardly from the terminal member and which may comprise an integral extension thereof, and a resiliently flexible magnetically permeable reed 13. The reed elements 9 and extend inwardly of the envelope from near the sealed ends thereof by such distances that their inner end portions are in overlapping relationship in the medial portion of the envelope.

Each reed 13 has its axially outer end anchored to the rebound post 12 of its reed element, and it extends lengthwise along its post at the front side thereof, that is, the side of the post that is adjacent to the other reed element. Each reed, moreover, is preloaded rearwardly against its post, with its inner tip portion under resilient flexing bias that normally urges it toward engagement with the post. When the reeds are thus preloaded against their respective posts, there is of course a gap 14 of a predetermined magnitude between the inner tip portions of the reeds, but when magnetic flux is caused to thread the reeds and the gap 14, the magnetic attraction force between the reeds overcomes their preloads and swings them forwardly away from their posts and into flatwise engagement with one another.

The opposing front surfaces of the tip portions of the two reeds are preferably coated as at 15 with substantially pure metallic tungsten to provide them with hard contacting surfaces which will withstand the mechanical stresses and impacts of the rather abrupt engagement to which the reeds are subjected at switch closure.

As explained in the above mentioned copending application, the reed elements are so oriented during assembly of the switch that the gap 14 is of a desired magnitude and the reeds engage with a true flatwise surface-to-surface contact, so that there is a maximum area of contact between them and the switch has low resistance when closed.

The reed that comprises a part of the reed element 9 carries a resilient auxiliary contactor 16 which normally engages the rear surface of said reed under forward flexing bias and is normally confined against its post 12 by the preload of its reed. The auxiliary contactor has a contact portion 17 that projects across the tip of said reed and forwardly beyond its front surface, part way across the gap 14, and which comprises the contact 7. As the reeds swing toward one another under the influence of a magnetic field, initial closure of the switch is effected by engagement of the auxiliary contactor against the reed of the reed element 10. Such engagement occurs before the tungsten coated surfaces 15 on the reeds themselves come into engagement. Likewise, when the magnetic field decays and the reeds swing back toward their posts, the opposing tungsten surfaces 15 on the two reeds separate first, and only thereafter does the auxiliary contactor separate from the reed of the reed element 10, to effect the actual opening of the switch.

As explained in said application, Ser. No. 714,490, the auxiliary contactor serves to prevent make bounce upon clo sure of the switch, and it insures a very clean and rapid separation of the contacts at switch opening. Inasmuch as the auxiliary contactor also provides one contact 7 of the contact pair 7, 8 that participates in the actual effective opening and closing of the switch, it is the auxiliary contactor that participates in the above discussed transfer and retransfer of material that occurs at switch opening, and hence the material comprising the contact 7 on the auxiliary contactor is of critical importance. With this in mind, the whole of the auxiliary contactor is preferably made of molybdenum wire.

The reed of the reed element 10 has that portion of its front surface which is engaged by the auxiliary contactor coated with substantially pure metallic tungsten to provide the other contact 8 with which the contact 7 on the auxiliary contactor engages.

Since the reed of the reed element 10 carries the tungsten member 8 of the tungsten molybdenum contact pair 7, 8, the terminal member 11 of that reed element is so connected in the circuit controlled by the switch 5 that it will be the negative switch terminal with reference to the direction of current flow at the time of switch opening; and the terminal member I l of the reed element 9, on which the molybdenum auxiliary contactor 16 is carried, is the positive terminal.

As explained above, it is of critical importance to long useful life of the switch that during the brief interval of initial switch opening the voltage across the contacts 7 and 8 should not attain a value at which ionization of tungsten occurs. To that end the present invention provides very simple voltage controlling circuit means 18 which is external to the switch and which is effective only during the time that the cathode spot obtains during switch opening. As illustrated in FIG. 1 that voltage controlling circuit means is shunted across the switch tenninals and comprises a resistor 19 and a capacitor 20 that are connected in series with one another. As illustrated in FIG. 2, wherein the switch controls an inductive load 22, the resistor 19 is connected in shunt across the load to thus control the peak voltage which appears across the load at the instant of switch opening and thereby control the voltage across the switch at that instant.

Thus the function of the resistor 19 is to control the voltage across the contacts of the switch at the instant of their initial separation. The function of the capacitor 20, when it is used, is to serve as a timed cutoff whereby the resistor is permitted to remain effective in the circuit during only the very brief but critical interval of initial separation of the switch contacts.

It is important that the resistance value of the resistor 19 be such as to keep the voltage across the contacts low enough to prevent tungsten ionization from occurring. But it should not be too low, for it must permit the voltage across the contacts to rise to a high enough value to allow the ionization of molybdenum by which the essential retransfer of that metal is effected. However, owing to the substantial difference between the vaporizing temperatures of tungsten and of molybdenum, the resistance value of the resistor need not be held to close tolerances.

Generally, for a switch with a tungsten molybdenum contact pair, the resistor 19 should have a resistance value on the order of 500 to 5,000 ohms. lfthe current to be interrupted by the switch is on the order to 0.5 amperes, the resistor 19 can have a value of 1,200 ohms and can be a one-fourth watt resistor with percenttolerance. Needless to say, such a resistor is very inexpensive.

With a substantially different value of current flowing in the load circuit at the instant of switch opening, some experimentation may be needed in order to determine the best value for the resistor 19. The particular load circuit controlled by the switch, as well as the current flowing therein at the moment of switch opening, determine the peak voltage across the switch contacts at the moment of their initial separation, and this peak voltage is difficult to calculate in most practical cases because it depends upon such factors as the distributed capacitance in the inductor of the load circuit, as well as upon the inductance of the circuit and the current flowing therein at the moment of switch opening. Therefore it is best to arrange a test setup of the particular switch and load circuit to be used and cycle it through a large number of switch operations, carefully observing the switch contacts from time to time. If the lower melting point metal (molybdenum) is building up on the negative switch contact, the voltage across the switch at the instant of contact separation is not high enough, and the value of the resistor 19 should be increased. if the switch contact of the higher melting temperature metal (tungsten) is breaking down, the voltage across the contacts at the instant of their separation is too high and the resistor 19 should be replaced by one of lower value.

The purpose of the capacitor 20, as indicated above, is to limit the time during which the resistor 19 is effective in the circuit, and thus to insure that the power drawn by the shunt circuit will not exceed a very low value such as will not interfere with desired operation of the load circuit. The capacity of the capacitor should be such that the time constant of the shunt circuit comprising the resistor i9 and said capacitor is on the order of one microsecond. it need not be longer; to insure satisfactory results it should not be substantially less. Thus with the 1,200 ohm resistor of the above example, the capacitor can have a capacitance of 800 micromicrofarads; and since its value need not be held with critical accuracy it can be a very inexpensive 20 percent tolerance capacitor.

The reason why the time constant of the shunt circuit can be so small is that the voltage surge which it is intended to limit is of extremely brief duration and tends to occur during an interval of somewhat less than one microsecond after initial separation of the contacts occurs. it should perhaps be pointed out that if this voltage surge were not properly limited it would initiate ionization that would result in substantial vaporization of tungsten because the voltage required to maintain ionization, once it is initiated, is substantially lower than the voltage needed to initiate it.

The circuit of FIG. 2 is one in which the switch 5 controls the connection of an inductive load 22 with a current source 23 which is illustrated as a battery. It will be understood that the negative terminal of the current source is connected with the tungsten contact of the switch 5, while the metal contact having the lower vaporizing temperature is connected with the positive terminal in series with the load.

If the inductive load has a low resistance, the voltage limiting circuit 18 need comprise only a resistor 19 connected in shunt across the load, and the capacitor 20 can be omitted, because with the switch 5 closed, the power drawn by the resistor 19 is a negligible percentage of the steady state power drawn by the load. Thus if the load 22 draws one-half amp. of current at 6 volts, and the resistor 19 has a value of L200 ohms, only 0.005 amp. will flow through the resistor while the switch is closed-about 1 percent of the power consumed by the load. At the instant of switch opening, however, the resistor limits maximum voltage across the switch terminals to 600 volts, which is below the critical value at which tungsten vaporization can occur.

If the load has a high resistance relative to the resistor 19, and loss of power through the resistor 19 cannot be tolerated, the capacitor 20 can be connected in series with the load and the resistor 19, as shown in FIG. 2, to limit the time during which the resistor is eflective in the circuit, the value of the capacitor being selected to afford a time constant which need not exceed about one microsecond.

The extent to which the simple expedient of this invention achieves its objective can be appreciated from the fact that a reed switch of the type herein illustrated and described by way of example, and with the above described circuit means connected across its terminals, was operated at 600 cycles per second, breaking an inductive load of 0.3 amps at about 3,300 v. (i.e., about 1 KW of power), with current always flowing in the same direction at the instant of switch opening. The test was tenninated after about 300 hours because of a failure in the load circuit. Although this test involved well over half a billion operations, the switch was still in useable condition at the end of it and was apparently capable of hundreds of thousands of additional operations under the same conditions.

In a subsequent test with another similar switch, connected in a circuit like that illustrated in FIG. 1, and with the switch breaking 0.55 amps. at 3,000 volts, the switch operated satisfactorily and consistently through almost two billion cycles of opening and closing.

From the foregoing description taken with the accompanying drawing, it will be apparent that this invention provides very simple and inexpensive external circuit means, connectable with a switch having contacts of unlike metal operating in a deep vacuum environment, for greatly prolonging the life of the switch.

Those skilled in the art will appreciate that the invention can be embodied in forms other than as herein disclosed for purposes of illustration.

The invention is defined by the following claims:

1. Switch means having a long useful life, for repeatedly opening an inductive load circuit in which current always flows in the same direction at the instant of switch opening, said switch means comprising, in combination:

A. an envelope evacuated to a deep vacuum;

B. a pair of contact members in said envelope, one of which is relatively movable toward and from contacting engagement with the other,

1. one of said contact members being of tungsten and being connectable with the terminal of the load circuit that is always negative at opening of the switch means, and

2. the other of said contact members being of another metal having lower melting and vaporizing temperaturcs than tungsten and being connectable with the other terminal of the load circuit; and

C. a resistor so connected with one of the contact members and with the load circuit as to control the voltage across the contact members at the instant of switch opening, and having a value of resistance such that the peak voltage across the contacs at the instant of their separation is high enough for vaporization of said other metal but not high enough for vaporization of tungsten.

2. The switch means of claim 1, further characterized by:

said resistor having a resistance value on the order of 500 to 5,000 ohms.

3. The switch means of claim 1, further characterized by:

means connected in series with said resistor for normally preventing flow of current therethrough at times when the switch means is closed.

4. The switch means of claim 3, wherein said means connected in series with the resistor comprises:

a capacitor having a capacitance such that the time constant of the resistor-capacitor circuit is not substantially less than lmlcrosecond.

5. The switch means of claim 1, further characterized by:

said other contact member being of molybdenum. 

1. Switch means having a long useful life, for repeatedly opening an inductive load circuit in which current always flows in the same direction at the instant of switch opening, said switch means comprising, in combination: A. an envelope evacuated to a deep vacuum; B. a pair of contact members in said envelope, one of which is relatively movable toward and from contacting engagement with the other,
 1. one of said contact members being of tungsten and being connectable with the terminal of the load circuit that is always negative at opening of the switch means, and
 2. the other of said contact members being of another metal having lower melting and vaporizing temperatures than tungsten and being connectable with the other terminal of the load circuit; and C. a resistor so connected with one of the contact members and with the load circuit as to control the voltage across the contact members at the instant of switch opening, and having a value of resistance such that the peak voltage across the contacts at the instant of their separation is high enough for vaporization of said other metal but not high enough for vaporization of tungsten.
 2. the other of said contact members being of another metal having lower melting and vaporizing temperatures than tungsten and being connectable with the other terminal of the load circuit; and C. a resistor so connected with one of the contact members and with the load circuit as to control the voltage across the contact members at the instant of switch opening, and having a value of resistance such that the peak voltage across the contacts at the instant of their separation is high enough for vaporization of said other metal but not high enough for vaporization of tungsten.
 2. The switch means of claim 1, further characterized by: said resistor having a resistance value on the order of 500 to 5,000 ohms.
 3. The switch means of claim 1, further characterized by: means connected in series with said resistor for normally preventing flow of current therethrough at times when the switch means is closed.
 4. The switch means of claim 3, wherein said means connected in series with the resistor comprises: a capacitor having a capacitance such that the time constant of the resistor-capacitor circuit is not substantially less than 1microsecond.
 5. The switch means of claim 1, further characterized by: said other contact member being of molybdenum. 